System and method for monitoring wear of braking frictional pad of motor vehicle

ABSTRACT

A method and a system for monitoring wear of a braking frictional pad of a motor vehicle, includes stopping the motor vehicle stably on a horizontal or substantially horizontal plane; determining a current fluid level in a brake fluid reservoir; determining a volume difference (ΔV L ) of the brake fluid in the brake fluid reservoir by comparing the determined current fluid level with a predetermined reference fluid level; when a thickness loss of a braking frictional pad equipped for a front vehicle wheel or a rear vehicle wheel is known, determining a thickness loss of a braking frictional pad assigned for the other front vehicle wheel or the other rear vehicle wheel based on the volume difference of the brake fluid in the brake fluid reservoir and a diameter of a brake piston of the respective brake device.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure claims the benefit of priority of co-pendingChinese Patent Application No. 202110539519.7, filed on May 18, 2021,and entitled “SYSTEM AND METHOD FOR MONITORING WEAR OF BRAKINGFRICTIONAL PAD OF MOTOR VEHICLE”, the contents of which are incorporatedin full by reference herein.

TECHNICAL FIELD

The present disclosure generally relates to a system and a method formonitoring wear of a braking frictional pad of a motor vehicle.

BACKGROUND

Brake devices are mandatory for motor vehicles under traffic laws andregulations. A brake device generally includes a brake disc installedsuch that it can be rotated together with a vehicle wheel's wheel rimand a braking frictional pad (which may be also referred to as a brakingfrictional plate) installed non-rotatably relative to the vehicle rim.For example, each brake disc can be equipped with two braking frictionalpads which are installed on opposing sides of the respective brake discwith gaps therebetween respectively. The two braking frictional pads canbe moved by a drive mechanism to contact the rotating brake disc, so asto clamp the brake disc. In this way, the brake disc can be stopped dueto friction braking.

Usually, the brake disc is harder than the braking frictional pads.Therefore, the braking frictional pad can be worn and thus have athinned thickness due to long-term braking. In order to generatesufficient friction braking, it is required to replace the old brakingfrictional pad, after its thickness is less than a given value, with anew one in time. Therefore, it is necessary to monitor the brakingfrictional pad's wear state.

A conventional method for monitoring the wear of a braking frictionalpad includes providing a metal sheet on a backing plate of the brakingfrictional pad. This metal sheet is configured to have a free endadjacent to a wear surface (i.e., a surface contacting the brake disc)of the braking frictional pad. Moreover, the free end is spaced from thewear surface by a given distance along a thickness direction of thebrake disc. Normally, the free end of the metal sheet is not in contactwith the brake disc when the braking frictional pad contacts the brakedisc to generate friction braking. However, after the braking frictionalpad is thinned to a given extent, the free end of the metal sheet willbe in contact with the rotating brake disc and thus a harsh sound willbe generated during the friction braking. Therefore, a driver is alertedthat the braking frictional pad shall be replaced with a new one. Thisconventional method for monitoring the wear state of the brakingfrictional pad is disadvantageous in that the driver cannot know thewear state of the braking frictional pad before the harsh sound isgenerated. Furthermore, contact of the metal sheet with the rotatingbrake disc can also damage the brake disc.

Another conventional method for monitoring the wear state of a brakingfrictional pad includes providing the braking frictional pad with aresistance sensor. For instance, the resistance sensor is configured tobe embedded directly in a frictional material of the braking frictionalpad or inserted into a backing plate of the braking frictional pad suchthat the sensor is distributed along a thickness direction of thefrictional material. When the braking frictional pad is worn and thusthinned to a given extent, a resistance value of the resistance sensorcan change during braking. Therefore, an early warning could be achievedby monitoring the change of the resistance value. However, this methodfor monitoring the wear state of the braking frictional pad isdisadvantageous in that the resistance sensor has to be equipped with aspecific cable so as to read an electrical signal from the sensor.However, the existence of such a specific cable increases the complexityof suspension structure design and assembling. Furthermore, such aresistance sensor cannot continuously monitor the wear state of thebraking frictional pad. Moreover, the resistance sensor and its specificcable can result in higher component costs and higher design andassembling costs.

Therefore, in order to reduce design and assembling costs, it is moreadvantageous to monitor the wear degree of the braking frictional padwithout a resistance sensor.

SUMMARY

In order to solve the aforementioned issues in prior art, the presentdisclosure proposes a novel system and method for monitoring wear of abraking frictional pad such that no additional sensor is required tomonitor the wear state of the braking frictional pads equipped for allvehicle wheels of a motor vehicle.

According to an aspect, the present disclosure proposes a method formonitoring wear of a braking frictional pad of a motor vehicle, themotor vehicle comprising vehicle wheels, a brake device acting on eachvehicle wheel, the brake device including a braking frictional padconfigured to be non-rotatable relative to the vehicle wheel and belinearly movable parallel to a rotational axis of the vehicle wheel, themotor vehicle also comprising a hydraulic brake drive and a brake fluidreservoir in fluid communication with the hydraulic brake drive, thehydraulic brake drive including a linearly movable brake pistonconfigured to act on the braking frictional pad to brake the vehiclewheel, the method comprising:

stopping the motor vehicle stably on a horizontal or substantiallyhorizontal plane;

determining a current fluid level in the brake fluid reservoir;

determining a volume difference (ΔV_(L)) of the brake fluid in the brakefluid reservoir by comparing the determined current fluid level with areference fluid level; and

when a thickness loss of a braking frictional pad equipped for a frontvehicle wheel or a rear vehicle wheel is known, determining a thicknessloss of a braking frictional pad equipped for the other front vehiclewheel or the other rear vehicle wheel based on the volume difference ofthe brake fluid in the brake fluid reservoir and a diameter of the brakepiston of the respective brake device.

In an embodiment, the thickness loss of the braking frictional padequipped for the other front vehicle wheel or the other rear vehiclewheel is determined using the following equation:

ΔL _(X)=(ΔV _(L-π) D ₀ ² ·ΔL _(0-γ))/(πD _(X) ²)

wherein ΔL_(x) is the thickness loss of the braking frictional pad to bedetermined, D₀ is a diameter of the brake piston associated with thebraking frictional pad having the known thickness loss, D_(x) is adiameter of a brake piston associated with the braking frictional padwith the thickness loss to be determined, ΔV_(L) is the volumedifference of the brake fluid in the brake fluid reservoir, γ is anempirical constant.

In an embodiment, the reference fluid level is a fluid level in thebrake fluid reservoir measured when the motor vehicle stays stably in ahorizontal or substantially horizontal plane and when the motor vehicleis installed with brand new braking frictional pads or brakingfrictional pads having known thickness for all vehicle wheels.

In an embodiment, after the reference fluid level has been measured andbefore any subsequent determination of fluid level in the brake fluidreservoir, no additional brake fluid is added into the brake fluidreservoir.

In an embodiment, the known thickness loss of the braking frictional padis determined by a resistance sensor disposed within the brakingfrictional pad.

In an embodiment, an electric parking brake or an electric mechanicalbrake is equipped for the braking frictional pad having the knownthickness loss.

In an embodiment, the electric parking brake or the electric mechanicalbrake comprises a drive motor, and a piston driving part driven by thedrive motor to be linearly movable; when the motor vehicle is braked,the brake piston is driven by the piston driving part to contact andmove the braking frictional pad.

In an embodiment, the method further comprises:

operating, after the motor vehicle has been parked, the drive motor tomove the piston driving part to a preset reference position;

continuing to operate the drive motor to move the piston driving partfrom the reference position to a stop position where the piston drivingpart drives the brake piston to contact the braking frictional pad,wherein the stop position corresponds to a parking braking position ofthe brake piston;

determining a distance between the reference position and the stopposition; and calculating a difference between the determined distanceand a predetermined reference distance to determine a thickness loss ofthe associated braking frictional pad as the known thickness loss.

In an embodiment, the reference distance is a travel distance of thepiston driving part from the reference position to the stop position,when the thickness of the associated braking frictional pad is known orthe associated braking frictional pad is brand new, and when the motorvehicle is in parking braking.

In an embodiment, the known thickness loss is absolute value of thedifference between the determined distance and the reference distance.

In an embodiment, the method further comprises alerting a driver of themotor vehicle to replace the braking frictional pad with a new one whenthe thickness loss of the braking frictional pads exceeds a threshold.

In an embodiment, the electric parking brake or the electric mechanicalbrake also comprises a reducer mechanism connected to an output shaft ofthe drive motor, and a linear movement mechanism driven by the reducermechanism, the reducer mechanism is configured to provide a lockfunction in braking to prevent the piston driving part from movingfreely, and the piston driving part is configured to be driven by thelinear movement mechanism to linearly move.

In an embodiment, the linear movement mechanism is a thread-screwmechanism that includes a core shaft connected to the output shaft ofthe drive motor, the core shaft comprising external threads on its outersurface, and the piston driving part is a threaded part having internalthreads that engage the external threads of the core shaft and isconfigured to be non-rotatable relative to the core shaft, and beaxially movable back and forth along the core shaft.

In an embodiment, the distance between the reference position and thestop position is at least dependent on a rotation speed of the outputshaft of the drive motor, a transmission ratio of the reducer mechanism,a pitch of thread of the thread-screw mechanism, and a time durationduring which the drive motor is turned on.

In an embodiment, before the known thickness loss is determined, parkingbraking of a vehicle wheel that is not equipped with the electricparking brake is carried out by the hydraulic brake drive.

In an embodiment, a direction along which the piston driving part movesto the reference position is opposite to a direction along which thepiston driving part moves from the reference position to the stopposition.

According to another aspect, the present disclosure proposes a systemfor monitoring wear of a braking frictional pad of a motor vehicle, themotor vehicle comprising vehicle wheels and a brake device acting oneach of the vehicle wheels, the brake device including a brakingfrictional pad configured to be non-rotatable relative to the vehiclewheel and be linearly movable parallel to a rotational axis of thevehicle wheel, the motor vehicle also comprising a hydraulic brake driveand a brake fluid reservoir in fluid communication with the hydraulicbrake drive, the hydraulic brake drive including a linearly movablebrake piston which is configured to act on the braking frictional pad tobrake the vehicle wheel, the system comprising:

a fluid level sensor provided in the brake fluid reservoir; and

an electronic control unit connected to the fluid level sensor toreceive fluid level data, comprising:

a first module configured to provide instructions for stopping the motorvehicle stably on a horizontal or substantially horizontal plane;

a second module configured to provide instructions for determining acurrent fluid level in the brake fluid reservoir;

a third module configured to provide instructions for determining avolume difference (ΔV_(L)) of the brake fluid in the brake fluidreservoir based on a preset reference fluid level and the determinedcurrent fluid level; and

a fourth module configured to provide instructions for, when a thicknessloss of a braking frictional pad associated with a front vehicle wheelor a rear vehicle wheel is known, determining a thickness loss of abraking frictional pad associated with the other front vehicle wheel orthe other rear vehicle wheel based on the volume difference of the brakefluid in the brake fluid reservoir and a diameter of the brake piston ofthe respective brake device.

In an embodiment, the fourth module is configured to determine thethickness loss of the braking frictional pad associated with the otherfront vehicle wheel or the other rear vehicle wheel using an equation asfollows:

ΔL _(X)=(ΔV _(L-π) D ₀ ² ·ΔL _(0-γ))/(πD _(X) ²)

wherein ΔL_(x) is the thickness loss of the braking frictional pad to bedetermined, D₀ is a diameter of the brake piston associated with thebraking frictional pad having the known thickness loss, D_(x) is adiameter of a brake piston associated with the braking frictional padwith the thickness loss to be determined, ΔV_(L) is the volumedifference of the brake fluid in the brake fluid reservoir, γ is anempirical constant.

In an embodiment, the reference fluid level is a fluid level in thebrake fluid reservoir, when the motor vehicle stops on a horizontal orsubstantially horizontal plane and the thickness of each brakingfrictional pad is known or each braking frictional pad is brand new.

In an embodiment, the fourth module generating the instructions is basedon that no additional brake fluid is added into the brake fluidreservoir.

In an embodiment, the braking frictional pad having the known thicknessloss is provided with a resistance sensor therein, so as to determinethe thickness loss of this braking frictional pad.

In an embodiment, an electric parking brake or an electric mechanicalbrake is equipped for the braking frictional pad having the knownthickness loss.

In an embodiment, the electric parking brake or the electric mechanicalbrake includes a drive motor and a piston driving part driven by thedrive motor to be linearly movable, the piston driving part beingconfigured to, when the motor vehicle is braked, be driven by the pistondriving part to contact the braking frictional pad, so that the brakingfrictional pad is driven to move.

In an embodiment, the electronic control unit includes:

-   a fifth module configured to generate instructions for operating,    after the motor vehicle has parked, the drive motor such that the    piston driving part is moved to a preset reference position;-   a sixth module configured to generate instructions for, after the    fifth module, continuing to operate the drive motor such that the    piston driving part is moved from the reference position to a stop    position where it contacts the brake piston, wherein the stop    position is associated with a parking braking position of the brake    piston; and-   a seventh module configured to generate instructions for determining    a distance between the reference position and the stop position and    comparing the determined distance with a preset reference distance    to determine a thickness loss of the monitored braking frictional    pad, as the known thickness loss.

In an embodiment, the reference distance is a travel distance of thepiston driving part moving from the reference position to the stopposition, which travel distance is determined in parking braking whenthe thickness of the monitored braking frictional pad is known or themonitored braking frictional pad is a brand new one.

In an embodiment, the known thickness loss of the monitored brakingfrictional pad is the absolute value of difference between thedetermined distance and the reference distance.

In an embodiment, the electronic control unit includes an eighth moduleconfigured to generate instructions for alerting a driver of the motorto replace the braking frictional pad with a new one when the thicknessloss of any one braking frictional pad to be monitored is greater than agiven value.

In an embodiment, the electric parking brake or the electric mechanicalbrake further includes a reducer mechanism connected to an output shaftof the drive motor, and a linear movement mechanism driven by thereducer mechanism, the reducer mechanism is configured to provide a lockfunction in braking so as to prevent the piston driving part fromarbitrarily moving, and the piston driving part is configured to bedriven by the linear movement mechanism through the drive motor tolinearly move.

In an embodiment, the linear movement mechanism is a thread-screwmechanism which includes a core shaft connected to the output shaft ofthe drive motor, the core shaft is provided with external threads on itsouter surface, the piston driving part is a threaded part havinginternal threads which engage the external threads and being configuredto be non-rotatable, when the core shaft rotates, and be axially movableback and forth along the core shaft.

In an embodiment, a direction along which the piston driving part movesto the reference position is opposite to a direction along which thepiston driving part moves from the reference position to the stopposition.

By using the technical measures provided in the present disclosure, thewear state of the braking frictional pad can be continuously monitoredwithout adding extra apparatus to the braking frictional pad. Therefore,this provides a reliable indicator for the motor vehicle's user toreplace the braking frictional pad and leads to lower costs ofdesigning, manufacturing, and assembling the brake device. Moreover,using the technical means provided in the present disclosure, it is notnecessary to equip the braking frictional pads of each vehicle wheel ofthe motor vehicle with an additional sensor or it only requires to equipall the braking frictional pads of the vehicle wheels on the single axlewith only one additional sensor, thus leading to lower vehicle designand assembling costs. Furthermore, the system and method according tothe present disclosure can be independently embodied or alternativelyembodied as a supplement to the convention method for monitoring thewear of the braking frictional pad using an additional sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The principles and other aspects of the present disclosure will beexplained in the following description with reference to the drawings,in which:

FIG. 1 is a schematic diagram illustrating a vehicle brake system inwhich a system or method for monitoring wear of a braking frictional padaccording to an embodiment of the present disclosure can be implemented,wherein the vehicle brake system includes a hydraulic brake driveconfigured to act on each vehicle wheel and an electric parking brakeconfigured to act on each rear wheel of a motor vehicle, and theelectric parking brake can be operated independently of the hydraulicbrake drive;

FIG. 2A is a schematic diagram illustrating an electric parking brake(EPB) according to one embodiment of the present disclosure, wherein theelectric parking brake is installed to a brake caliper of the motorvehicle, and a drive element of the electric parking brake for driving abrake piston is in a position where it is in contact with the brakepiston, such as a stop position where the braking frictional pad isdriven by the brake piston to contact a brake disc to implement parkingbraking;

FIG. 2B is a schematic diagram illustrating the electric parking brakeof FIG. 2A, wherein the drive element of the electric parking brake isin a start position, i.e., a reference position, of the drive element'sfree stroke before it contacts the brake piston;

FIG. 3 is a schematic diagram illustrating a curve graph of the currentchanged over time of a drive motor of the electric parking brake inoperation;

FIG. 4A is a schematic diagram illustrating a brake disc and two brakingfrictional pads which act on opposing sides of the brake discrespectively, wherein the motor vehicle is not in a parking brakingstate;

FIG. 4B is a schematic diagram illustrating the brake disc and the twobraking frictional pads of FIG. 4A, wherein the motor vehicle is in theparking braking state;

FIG. 5 is a flow chart illustrating a method according to an embodimentof the present disclosure for monitoring wear of a braking frictionalpad;

FIG. 6 is a block diagram illustrating a system according to anembodiment of the present disclosure for monitoring wear of a brakingfrictional pad;

FIG. 7A is a schematic diagram illustrating how parking braking iscarried out by the brake piston being driven by the hydraulic brakedrive, wherein the respective braking frictional pads are brand newones;

FIG. 7B is a schematic diagram illustrating how parking braking iscarried out by the brake piston being driven by the hydraulic brakedrive, wherein the respective braking frictional pads are used ones,i.e., they have been used after long mileage of the motor vehicle;

FIG. 8 is a flow chart illustrating a method according to anotherembodiment of the present disclosure for monitoring wear of a brakingfrictional pad; and

FIG. 9 is a block diagram illustrating a system according to anotherembodiment of the present disclosure for monitoring wear of a brakingfrictional pad.

DETAILED DESCRIPTION OF EMBODIMENTS

In the drawings of the present disclosure, the features having the sameconfiguration or same functions are represented by the same referencenumerals respectively.

FIG. 1 is a schematic diagram illustrating a vehicle brake systemaccording to an embodiment. The vehicle brake system includes ahydraulic brake drive configured to act on each vehicle wheel of a motorvehicle and an electric parking brake configured to act on only two rearvehicle wheels of the motor vehicle. The electric parking brake isconfigured to operate independently of the hydraulic brake drive. Itshould be understood by a person skilled in the art that in analternative embodiment, the electric parking brake of FIG. 1 can beconfigured to act on two front vehicle wheels of the motor vehicle. FIG.1 shows an embodiment of the present disclosure, in which the electricparking drive acts on the two rear vehicle wheels in an illustrative butnon-limiting manner. In the context of the present disclosure, the motorvehicle includes but is not limited to a combustion engine vehicle, anelectric vehicle, a hybrid electric vehicle or the like.

As shown, the motor vehicle includes an electronic control unit (ECU)100 configured for receiving or transmitting signals from or to relevantcomponents and a brake pedal 200. For instance, the brake pedal 200 canbe disposed in a passenger compartment of the motor vehicle such that itcan be pressed by a driver seating in the passenger compartment asneeded. For instance, the motor vehicle includes four vehicle wheels,i.e., a front left vehicle wheel FL, a front right vehicle wheel FR, arear left vehicle wheel RL, and a rear right vehicle wheel RR. Each ofthe front left vehicle wheel FL, the front right vehicle wheel FR, therear left vehicle wheel RL, and the rear right vehicle wheel RR isequipped with a brake device. For instance, the brake device of thefront left vehicle wheel FL includes a brake disc 500FL configured torotate with the front left vehicle wheel FL and two braking frictionalpads 610FL and 620FL, which are installed non-rotatably relative to thebrake disc 500L at opposing sides of the brake disc 500L respectively.The two braking frictional pads 610FL and 620FL can be installed on abrake caliper which at least partially surrounds the brake disc 500FLradially. When the motor vehicle is not braking, the braking frictionalpads 610FL and 620FL are each spaced from the brake disc 500L. Thebraking frictional pad 620FL is located on the side of the brake disc500FL facing the front left vehicle wheel FL, and the braking frictionalpad 610FL is located on the opposite side of the brake disc 500FL. Tobrake the motor vehicle, the braking frictional pads 610FL and 620FL canbe driven to move towards the brake disc 500FL respectively and clampthe brake disc such that they are in contact with the brake disc 500FLrotating with the front left vehicle wheel FL. This contact generates africtional force and increases the resistance to the vehicle wheel ofmotor vehicle, which facilitates to stop the motor vehicle. Similar tothe brake device of the front left vehicle wheel FL, each of the frontright vehicle wheel FR, the rear left vehicle wheel RL, and the rearright vehicle wheel RR is also equipped with a brake device. Therespective brake device of the respective vehicle wheel includes a brakedisc, such as 500FR, 500RL, or 500 RR, configured to rotate with therespective vehicle wheel and two braking frictional pads such as 610FRand 620FR, 610RL and 620RL, or 610RR and 620RR, which are installednon-rotatably relative to the respective brake disc at opposing sides ofthe respective brake disc respectively.

A hydraulic brake drive of the vehicle brake system includes a mainbrake pump 300. This main brake pump 300 is operatively connected to thebrake pedal 200. In the meanwhile, the main brake pump 300 is connectedvia hydraulic pipelines to brake wheel cylinders 310FL, 310FR, 310RL,and 310RR equipped for the vehicle wheels FL, FR, RL, and RRrespectively. Furthermore, a brake fluid reservoir 400 is also incommunication with the pump and/or the wheel cylinders via hydraulicpipelines. Each wheel cylinder can include a cylinder body and a brakepiston such that the brake piston is hydraulically movable in thecylinder body in a linear manner. Each of the brake wheel cylinders310FL, 310FR, 310RL, and 310RR is installed at a respective brakecaliper to drive the paired braking frictional pads to move towards therespective brake disc respectively. In this way, when the brake pedal200 is pressed, the main brake pump 300 drives a brake fluid via thehydraulic pipelines to operate the brake wheel cylinders 310FL, 310FR,310RL, and 310RR such that the respective braking frictional pads can bein contact with the respective brake discs and thus the motor vehiclecan be stopped or slowed down. Under control of the electronic controlunit 100, the brake wheel cylinders of the front vehicle wheels FL, FRor the rear vehicle wheels RL, RR can be operated independently of eachother.

Further as shown in FIG. 1, the rear left vehicle wheel RL is equippedwith an electric parking brake 700RL, and the rear right vehicle wheelRR is equipped with an electric parking brake 700RR. Each of theelectric parking brakes 700RL and 700RR is electrically connected to andcontrollable by the electronic control unit 100. Moreover, each of theelectric parking brakes 700RL and 700RR is installed at the respectivebrake caliper to act on the respective braking frictional pads. Theelectric parking brakes 700RL and 700RR can be operated independently ofthe respective brake wheel cylinders 310RL and 310RR such that after themotor vehicle has been stopped, the braking frictional pads of the rearleft vehicle wheel RL and/or the rear right vehicle wheel RR can bedriven by the corresponding brake wheel cylinder to move towards andcontact the respective brake disc. In this way, parking braking of themotor vehicle is realized by a frictional force generated between thebraking frictional pads and the brake disc(s).

For example, FIG. 2A shows one electric parking brake that can be theelectric parking brake 700RL or 700RR of FIG. 1. It should be understoodthat the drawings attached here are given for the purpose of explainingthe principle of the present disclosure only, but do not limit thedesign or implementation of any specific component therein.

The electric parking brake is installed relative to a brake caliper 630.The brake caliper 630 can be installed such that it at least partiallysurrounds the brake disc 500RL or 500RR. The brake caliper 630 isinstalled fixedly to the motor vehicle's body or its suspension. Theelectric parking brake includes a drive motor 311 installed fixedly tothe brake caliper 630. The electric parking brake further includes acore shaft 316 installed rotatably relative to the brake caliper 630. Athreaded part 317 is sleeved about an outer surface of the core shaft316 such that the threaded part is non-rotatable relative to the coreshaft but is only axially movable back and forth along the core shaft.External threads are formed on the outer surface of the core shaft 316,and internal threads are formed on an inner surface of a central hole ofthe threaded part 317 such that the external threads are able to engagewith the internal threads. Furthermore, the threaded part 317 is locatedwithin a hollow chamber of a brake piston 319 or is movableindependently of the brake piston 319. In the meanwhile, the brakepiston 319 is guided in a cylinder body 320 in such a way that the brakepiston 319 is movable linearly therein. The cylinder body 320 is fixedrelative to the brake caliper 630. It should be understood by a personskilled in the art that the brake piston 319 and the cylinder body 320mentioned here can function as the brake piston and the cylinder body ofthe brake wheel cylinders described above. In fact, a piston drivingforce generated by the electric parking brake and a piston driving forcegenerated by the main brake pump 300 via the respective brake wheelcylinder can be applied onto the brake piston 319 independently of eachother. When desired, interaction between the electric parking brake andthe brake piston 319 can be uncoupled. In the context of the presentdisclosure, the term “uncouple” means that two elements or devices donot generate a driving force applied therebetween.

The threaded part 317 can be guided linearly without rotating its outersurface. In other words, the threaded part 317 can be guided linearlywithin the hollow chamber of the brake piston 319 such that the threadedpart or its outer surface is non-rotatable. Therefore, the threaded part317 and the core shaft 316 constitute a thread-screw mechanism or theyform a thread-screw mechanism therebetween, such that the threaded part317, which does not rotate with the core shaft 316, is movable axiallyback and forth along the axis of the core shaft 316 due to the threadedengagement between the threaded part and the core shaft. Therefore, forexample, when parking braking is carried out, the threaded part 317contacts the brake piston 319 by the linear movement of the threadedpart 317 and further drives the brake piston 319 to push a respectivebraking frictional pad. In this way, the braking frictional padsarranged opposite to each other can be driven by the brake caliper 630to contact a respective brake disc and the braking frictional pads canbe pressed to generate on the brake disc a frictional force for parkingbraking. In this case, the brake piston 319 is in a parking brakingposition.

Because of the structural characteristics of the thread-screw mechanism,when the core shaft 316 is rotated reversely, the threaded part 317,which has contacted the brake piston 319 in the parking brakingposition, can be linearly moved reversely such that no force will beapplied by the threaded part 317 onto the brake piston 319. That is tosay, the electric parking brake is uncoupled from the brake piston 319.A travel distance of the threaded part 317 reversely moving during theuncoupling can depend on the time during which the core shaft 316 isreversely rotated or a mechanical stopper feature of the thread-screwmechanism itself. Usually, in order to release parking braking, it isonly required to reversely rotate the core shaft to an extent in whichthe threaded part 317 is no longer in contact with the brake piston 319so as to ensure that the frictional force for parking braking is nolonger applied by the braking frictional pads onto the brake disc. Inthis way, during the subsequent vehicle driving, the hydraulic brakedrive can quickly drive the brake piston, responsive to braking needs,such that the braking frictional pads can be enabled to contact thebrake disc again to apply thereon a frictional force for drivingbraking. In an example of the present disclosure, the thread-screwmechanism is a type of linear movement mechanism that can be driven by areducer mechanism as described below to drive the threaded part 317 tomove linearly. It should be understood by a person skilled in the artthat any other suitable means for converting a rotating movement into alinear movement can be adopted as the linear movement mechanism in thetechnical solutions of the present disclosure.

As shown in FIG. 2, the electric parking brake includes a reducermechanism 315 installed relative to the brake caliper 630. The reducermechanism 315 has an input end connected to an output shaft of the drivemotor 311, and an output end connected to the core shaft 316 such thatwhen the output shaft of the drive motor 311 rotates forwards orbackwards, the core shaft 316 can be driven by the reducer mechanism 315to rotate forwards or backwards correspondingly. As an example, thereducer mechanism can be a two-stage worm and screw mechanism installedon the brake caliper 630. The two-stage worm and screw mechanismincludes a first-stage reducer formed by a first-stage screw and afirst-stage worm in engagement with each other, and a second-stagereducer formed by a second-stage screw and a second-stage worm inengagement with each other. The first-stage screw is connected to theoutput shaft of the drive motor 311 and the second-stage worm isconnected to the core shaft 316. Both the first-stage worm and thesecond-stage screw are disposed on a single rotatable shaft. The reducermechanism is configured to provide a lock function during parkingbraking, i.e., such that a counterforce caused by the braking frictionalpads to the brake piston 317 will not be transmitted via the threadedpart 317, the core shaft 316, and the reducer mechanism to the outputshaft of the drive motor 311, causing the output shaft to rotate. Itshould be understood by a person skilled in the art that any other formof reducer mechanism for providing a similar lock function for parkingbraking such as a single-stage worm and screw mechanism or a gearreducer mechanism can be used in the technical solutions of the presentdisclosure.

The threaded part 317 can be called as a piston driving part 317 of theelectric parking brake. Therefore, during parking braking, if a pitch ofthread of the thread-screw mechanism, a reduction ratio of the reducermechanism, a run time of the output shaft of the drive motor 311 areknown, a travel distance of the piston driving part 317 moving from aposition where it is out of contact with the brake piston 319 to aposition where it pushes the brake piston 319 (i.e., being associatedwith the parking braking position of the brake piston 319) can becalculated.

FIG. 3 is a schematic diagram illustrating the changes over time of thecurrent of the drive motor 311 of the electric parking brake inoperation. When the electric parking brake is working, an operatingcurrent of the drive motor 311 can be measured by a specific detectioncircuit and the measured value can be transmitted into the electroniccontrol unit 100. When parking braking is required, the drive motor 311is activated to start. Because of the characteristics of the motor, agreater starting current will usually occur, which is shown as a currentpeak A in FIG. 3. After the current peak A, the operating current dropsquickly. Then the piston driving part 317 is driven by the drive motor311 through the reducer mechanism and the thread-screw mechanism toenable the piston driving part 317 to undergo a free stroke (duringwhich the piston driving part 317 is not in contact with the brakepiston 319 yet), the operating current of the drive motor 311 appearsstable, i.e., in a section from about t_(A) to t_(C). Then, at timet_(C), the piston driving part 317 begins to contact the brake piston319 and presses the brake piston 319 to drive a respective brakingfrictional pad to move towards and contact the brake disc. During thisperiod, as the piston driving part 317 will be correspondinglycounteracted and thus blocked by the brake piston 319, the operatingcurrent of the drive motor 311 rises quickly as shown in FIG. 3.Finally, when the brake piston 319 arrives at its parking brakingposition, a cut-off current peak B (at time t_(B)) will be set toprevent the drive motor from being damaged by an excessive operatingcurrent. When the operating current is equal to the cut-off current peakB, the drive motor 311 will be deactivated to stop working. Therefore,at time t_(B), because of the lock function of the reducer mechanism asmentioned above, the brake piston 319 is held in the parking brakingposition via the piston driving part 317 and the reducing mechanism suchthat parking braking is achieved by the electric parking brake. Whenvehicle driving is needed, the drive motor 311 is started again suchthat its output shaft is rotated reversely such that the piston drivingpart 317 can be linearly moved reversely to an extent in which the brakepiston 319 is released from its parking braking position, and africtional force for parking braking will no longer be generated by theelectric parking brake between the respective braking frictional padsand the brake disc. That is to say, the electric parking brake isuncoupled from the brake piston 319.

Therefore, it can be seen that when a motor vehicle of the prior art isbraking for parking, an initial position of its piston driving part(like 317) for parking braking is always random due to continuous wearof braking frictional pads. Therefore, in order to unify the randominitial positions and to determine a travel distance of the pistondriving part 317 moving linearly during parking braking, an initialposition can be intentionally set for the piston driving part 317.Generally, in the design of the thread-screw mechanism between thepiston driving part 317 and the core shaft 316, a stopper feature suchas a stopper block can be provided at an axial location of the coreshaft 316 so as to define the maximum extent to which the piston drivingpart 317 can move in a direction away from the brake piston 319, asshown in FIG. 2B. For an assembled electric parking brake, the maximummovable range of the piston driving part 317 is always fixed and thuscan be regarded as a reference position of the piston driving part 317.That is to say, whenever braking parking is required, the piston drivingpart 317 is set to first move to the reference position and then thetime taken by the piston driving part 317 to move from that referenceposition to a position, where the piston driving part 317 drives thebrake piston 319 to arrive at the parking braking position, is measured.Using the measured time, a distance of the brake driving part 317 movingfrom the reference position to a stop position (being associated withthe parking braking position) can be calculated.

It should be understood by a person skilled in the art that the timet_(A) of the current peak, the time t_(C) of stable conversion, and thetime t_(B) of the cut-off current peak can be detected by a suitableelectronic circuit, and transmitted to and recorded in the electroniccontrol unit 100. Therefore, using a program preset in the electroniccontrol unit 100, the changes of the current over time are recordedduring each parking braking. Then, by measuring or determining thetravelling time period of the piston driving part 317, a travel distanceL of a braking frictional pad from an initial position to a positionwhere parking braking is achieved can be determined by an equation (1):

L=f1(Δt _(E) ,η,p)   (1)

wherein L is a travel distance of a single braking frictional pad,Δt_(E) is a time period during which the piston driving part 317 of theelectric parking brake moves from its initial position to its stopposition (being associated with the parking braking position of thebrake piston 319), η is a transmission ratio of the reducer mechanism, pis a pitch of thread of the thread-screw mechanism, and f1 is a functionassociated with Δt_(E), η, and p. It should be understood by a personskilled in the art that although Δt_(E) can be equal to |t_(A)−t_(B)| inan example as further described below, this example is illustrative andnot in any way limiting . In another example, Δt_(E) can be equal to|t_(A)−t_(C)|. That is to say, it is okay to set a transition currentpoint C or a cut-off peak point B as the parking braking position forthe brake piston 319.

By way of an example, f1=(n_(M)/η)·p·Δt_(E)/2, wherein n_(M) is a rateof rotation of the output shaft of the drive motor 311. As anotherexample, a correction factor related to other aspects such as athickness loss of the brake disc or the like could also be added to thefunction of f1. For instance, a correction factor related to thethickness loss of the brake disc can be determined in advance throughexperiments. In this way, the function can be altered asf1=(n_(M)/η)·p·Δt_(E)/2+θ, wherein θ is the correction factor related tothe thickness loss of the brake disc.

Assume that the transition current point C and the cut-off peak point Brepresent the currents of a drive motor 311 with new braking frictionalpads, and a transition current point C′ and the cut-off peak point B′represent the currents of the drive motor 311 with regard to the samebraking frictional pads after they have been used for a certain period.As shown in FIGS. 4A and 4B (for clarity, the gaps between the pads anda brake disc are enlarged for illustration), after the new brakingfrictional pads are installed in place, the electric parking brake isactivated to record the travel time period, Δt_(E)=|t_(A)−t_(B)|, duringwhich the piston driving part 317 moves from its initial position to itsstop position for parking braking, and a travel distance (i.e., areference distance) L_(O) of the piston driving part 317 can bedetermined according to the above equation (1). Then, after the brakingfrictional pads are worn due to usage, parking braking is carried outagain. To determine the present thickness of the braking frictional pad,the electric parking brake is first activated during the parking brakingto drive the piston driving part 317 to its reference position, and thenthe drive motor 311 of the electric parking brake is reversely activatedsuch that the piston driving part 317 is moved from the referenceposition to its stop position by a distance L_(A), which corresponds toa time period, Δt_(E)=|t_(A)−t_(B′)|. Because the above calculationprocess can be carried out whenever parking braking is achieved, thetime t_(A) of a starting current corresponding to the starting currentpeak A is always unchanged under the premise that the performance of thedrive motor 311 is unchanged. Therefore, it is proper to compare L_(O)of the initial calculation with L_(A) of each subsequent calculationthat has the same time point for the starting current. That is to say,ΔL_(R)=|L_(O)−L_(A)| can be regarded as the thickness loss of thebraking frictional pad. In an embodiment of the present disclosure, atravel distance of the piston driving part 317 first determined withregard to a new braking frictional pad or an average travel distance ofthe piston driving part 317 calculated for several times during itsinitial usage period can be considered as the reference distance L_(O)of the piston driving part 317.

The method for determining the thickness loss of the braking frictionalpad according to the present disclosure as explained above isadvantageous for the following reasons. Although a time period|t_(B)−t_(C)| for parking braking using new braking frictional pads isless than a time period |t_(B′)−t_(C′)| for parking braking using thebraking frictional pads after a period of usage, the difference betweenthem is very tiny. If such a tiny time difference were used to make thecalculation, greater calculation errors would be introduced, and itwould be required to determine the point C with very high precision.However, according to the present disclosure, whenever parking brakingis carried out, the piston driving part 317 is first returned to itsreference position. This results in a uniform way to calculate the timedifference between the time t_(A) of the starting current of the drivemotor 311 and the time t_(B) or t_(B′) of the cut-off current peak andto determine a travel distance of the piston driving part 317 based onsuch time difference. As the difference between the time t_(A) of thestarting current and the time t_(B) or t_(B′) of the cut-off currentpeak is significantly greater than the difference between the transitioncurrent time t_(C) or t_(C′) and the time t_(B) or t_(B′) of the cut-offcurrent peak, calculation errors can be reduced and thus the ultimatecalculation accuracy can be increased.

Therefore, with respect to a motor vehicle equipped with an electricparking brake, the present disclosure proposes a novel method or processfor continuously monitoring the thickness loss (or wear) of a brakingfrictional pad cooperating with the electric parking brake.

FIG. 5 is a flow chart illustrating an example of the method or processfor monitoring the wear of the braking frictional pad. It should beunderstood by a person skilled in the art that any method and process ortheir steps explained herein can be coded as programs that can be storedin a memory of the electronic control unit 100 and retrieved andexecuted as required. In step S10, it is determined whether or not themotor vehicle has stopped stably. For instance, whether the vehicle hasstopped stably is determined based on a collection of information fromvarious existing sensors of the motor vehicle. If the determinationresult in step S10 is “Yes”, the method or process goes to step S14;otherwise, the method or process ends. In step S14, for safety reasonsduring parking braking, a wheel cylinder of a vehicle wheel that is notequipped with the electric parking brake is first activated such that itwill ensure that the vehicle wheel that is not equipped with theelectric parking brake is always in braking. Then, in step S15, theelectric parking brake is activated such that its drive motor 311 candrive the piston driving part 317 to operate in such a way that thepiston driving part 317 goes to its reference position and stays in thereference position. Taking the electric parking brakes 700RL and 700RRof FIG. 1 as an example of the electric parking brake as mentioned inthe method or process, in step S20, the electric parking brake 700RLand/or 700RR of FIG. 1 is operated in such a way that the respectivebrake piston will drive the respective braking frictional pad to contactthe respective brake disc to accomplish parking braking. Then, a traveldistance of each braking frictional pad of the rear wheel RL or RRequipped with the electric parking brake 700RL or 700RR duringmonitoring of the wear of the braking frictional pad through parkingbraking can be calculated using the equation (1) and based on a run timeΔt of the drive motor 311, a transmission ratio η of the relevantreducer mechanism, a pitch of thread of the relevant thread-screwmechanism. For instance, Δt of the drive motor 311 can be Δt_(E) fordetermining L_(O) or L_(A) as shown in FIGS. 4A and 4B.

Next, in step S30, it will be determined whether it is the firstcalculation for parking braking or not. If the determination result is“Yes”, then it means that the calculation result of step S20 can bereferred to as L_(O). The calculation result will then be recorded asthe reference distance L_(O) for later use. Alternatively, this step canbe carried out when a new motor vehicle just leaves a production line ofa vehicle plant or when tire replacement for a motor vehicle is carriedout in maintenance. If the determination result is “No”, then it meansthat the calculation result of step S20 can be regarded as L_(A) andthus a thickness loss of the current braking frictional pad can bedetermined by the absolute value of difference between the traveldistance L_(A) from the present calculation for monitoring the wear ofthe braking frictional pad and the reference distance L_(O) from thefirst calculation.

Next, in step S60, if the determined thickness loss of the currentbraking frictional pad is greater than or equal to a given value, themethod or process goes to step S70. For instance, in step S70, anindicator in a passenger compartment can be activated so as to visuallyand/or audibly alert a driver sitting in the passenger compartment;and/or some functions of the motor vehicle can be limited, for example,the motor vehicle will be configured to run only at a restricted speedsuch as a maximum speed of 50 kilometers per hour, so as to alert thedriver. If the determined thickness loss of the current brakingfrictional pad is less than the given value, the method or process goesto step S80 and calculation data can be sent to a dashboard or thedriver's mobile device to be displayed as status information. In stepS80, the electric parking brake can continue to work so as to ensurethat the motor vehicle parks safely or the wheel cylinder of the vehiclewheel that is not equipped with the electric parking brake can beselectively deactivated.

In the above embodiment, although the principles of the presentdisclosure are explained with respect to the electric parking brakes700RL and 700RR of the rear vehicle wheels RL and RR, it should beunderstood by a person skilled in the art that these basic ideas arealso applicable to a motor vehicle which is equipped with an electricparking brake only for its front vehicle wheel FL and/or FR.

In an alternative embodiment, the principles of the present disclosureare applicable to a motor vehicle equipped with an electric mechanicalbrake (EMB). In the context of the present disclosure, the electricmechanical brake comprises a device that can drive a brake piston togenerate a frictional braking force between a braking frictional pad anda brake disc in both driving braking and parking braking. The electricmechanical brake can be applied to each of the four vehicle wheels of amotor vehicle. Moreover, the motor vehicle equipped with the electricmechanical brake can be also provided with a hydraulic brake drive as abraking safety auxiliary device, or without the hydraulic brake drivesuch that driving braking and parking braking can be accomplished by theelectric mechanical brake only. It should be understood by a personskilled in the art that the electric mechanical brake further includes adrive motor, a reducer mechanism operatively connected to the drivemotor and having a lock function in braking, and a piston driving partoperatively connected to the reducer mechanism and configured to act ona brake piston to drive relevant braking frictional pads to move.

Therefore, the aforementioned method or process is also applicable to amotor vehicle equipped with electric mechanical brakes. For instance,for a motor vehicle equipped with electric mechanical brakes only, stepS10 of FIG. 5 can be modified such that braking of front vehicle wheelsis firstly accomplished by the associated electric mechanical brakes. Inthe subsequent steps, the wear of the braking frictional pads can bedetermined only with respect to rear vehicle wheels via the electricmechanical brakes thereof; and then parking braking is accomplished bythe electric mechanical brakes of the rear vehicle wheels only; and inthe meanwhile, the wear of the braking frictional pads can be determinedwith respect to the front vehicle wheels via the electric mechanicalbrakes thereof.

As discussed above, the present disclosure provides a method or processfor monitoring wear of a braking frictional pad of a motor vehicle,wherein the motor vehicle includes a brake device acting on each vehiclewheel of two front vehicle wheels or two rear vehicle wheels. The brakedevice has a brake disc configured to rotate with the correspondingvehicle wheel, a braking frictional pad configured to be non-rotatablerelative to the vehicle wheel and linearly movable parallel to arotational axis of the vehicle wheel, a cylinder body installed to themotor vehicle's body, and a brake piston linearly movable in thecylinder body. The motor vehicle further includes an electric parkingbrake, or an electric mechanical brake equipped for each vehicle wheelof the two front vehicle wheels or the two rear vehicle wheels. Theelectric parking brake or the electric mechanical brake includes a drivemotor, a reducer mechanism connected to an output shaft of the drivemotor, a linear movement mechanism driven by the reducer mechanism, anda piston driving part driven by the linear movement mechanism to movelinearly. The reducer mechanism is configured to provide, when braking,a lock function by which the piston driving part is prevented frommoving freely. The brake piston is configured to be driven by the pistondriving part to contact and move the braking frictional pad.

The method or process includes:

-   after the motor vehicle has been parked, operating the drive motor    to drive the piston driving part to a preset reference position;-   continuing to operate the drive motor to drive the piston driving    part from the reference position to a stop position where it    contacts the brake piston, wherein the stop position corresponds to    a parking braking position of the brake piston;-   determining a distance between the reference position and the stop    position; and comparing the determined distance with a preset    reference distance to determine the wear of the braking frictional    sheet to be monitored.

Furthermore, the present disclosure provides a system for monitoringwear of a braking frictional pad of a motor vehicle, wherein the motorvehicle includes a brake device acting on each vehicle wheel. The brakedevice has a brake disc configured to rotate with the correspondingvehicle wheel, a braking frictional pad configured to be non-rotatablerelative to the vehicle wheel and linearly movable parallel to arotational axis of the vehicle wheel, a cylinder body secured relativeto the motor vehicle's body, and a brake piston linearly movable in thecylinder body. The motor vehicle further includes a brake deviceequipped for each vehicle wheel of two front vehicle wheels or two rearvehicle wheels. The brake device has a brake disc rotatable with thecorresponding vehicle wheel and a braking frictional pad non-rotatablerelative to the relevant vehicle wheel and linearly movable parallel toa rotational axis of the corresponding vehicle wheel.

Furthermore, as shown in FIG. 6, the system includes:

-   an electric parking brake 700 or an electric mechanical brake 710    equipped for each vehicle wheel of the two front vehicle wheels or    the two rear vehicle wheels, wherein the electric parking brake or    the electric mechanical brake includes a drive motor, a reducer    mechanism connected to an output shaft of the drive motor, a linear    movement mechanism driven by the reducer mechanism, and a piston    driving part driven by the linear movement mechanism to move    linearly; the reducer mechanism is configured to provide, when    braking, a lock function by which the piston driving part is    prevented from moving freely; the brake piston is configured to be    driven by the piston driving part to contact and move the braking    frictional pad; and-   an electronic control unit 100 which is electrically connected to    the electric parking brake 700 or the electric mechanical brake 710,    to control operation of the electric parking brake 700 or the    electric mechanical brake 710.

The electronic control unit 100 includes a first module 1001 configuredto provide, after the motor vehicle has been parked, instructions foroperating the drive motor such that the piston driving part is moved toa preset reference position;

-   a second module 1002 configured to provide instructions for    continuing to operate the drive motor to drive the piston driving    part from the reference position to a stop position where it    contacts the brake piston, wherein the stop position corresponds to    a parking braking position of the brake piston;-   a third module 1003 configured to provide instructions for    determining a distance between the reference position and the stop    position; and-   a fourth module 1004 configured to provide instructions for    comparing the determined distance with a preset reference distance    to determine the wear of the braking frictional sheet to be    monitored.

The reference distance is a travel distance of the piston driving partmoving from the reference position to the stop position when thethickness of the monitored braking frictional pad is known or when themonitored braking frictional pad is a brand new braking frictional pad.In the above embodiments, the reference distance is determined when thebraking frictional pad is a brand new one. However, it should beunderstood by a person skilled in the art that the determination of thereference distance can be based on the known thickness of the monitoredbraking frictional sheet. That is to say, if the thickness of themonitored braking frictional pad is known, the reference distance can bedetermined in advance. Understandably, when the braking frictional padis a brand new one, its thickness is known. If the monitored brakingfrictional pad is a used one, it is required to measure and record thethickness of the monitored braking frictional pad and thencorrespondingly determine the reference distance in advance to apply themethod or process or the system according to the present disclosure.

A thickness loss of the monitored braking frictional pad is the absolutevalue of the difference between the determined distance and thereference distance.

In one embodiment, each vehicle wheel of the motor vehicle is equippedwith the electric mechanical brake only.

The electronic control unit 100 further includes a fifth module 1005configured to provide instructions for alerting a driver of the motorvehicle to replace the monitored braking frictional pad with a new onewhen the thickness loss of the monitored braking frictional pad isgreater than a preset threshold. For instance, the electronic controlunit 100 can be connected to a buzzer alarm, which will beep after itreceives the instructions from the fifth module 1005. For a motorvehicle equipped with electric parking brakes at its front or rearvehicle wheels only, this preset value can be determined by anautomobile manufacturer based on the collective historical data ofreplacing braking frictional pads by vehicle users.

The linear movement mechanism is a thread-screw mechanism including acore shaft connected to an output shaft of the reducer mechanism. Thecore shaft is formed with external threads on its outer surface. Thepiston driving part is a threaded part having internal threads which areconfigured to engage the external threads. Moreover, when the core shaftrotates, the threaded part is configured to be non-rotatable and isdriven to axially move back and forth along the core shaft.

The distance between the reference position and the stop position is atleast dependent on a rotation speed of an output shaft of the drivemotor, a transmission ratio of the reducer mechanism, a pitch of threadof the thread-screw mechanism, and a duration of the drive motor beingturned on.

If the motor vehicle is equipped with the electric parking brake only onthe front or rear wheels, the motor vehicle further includes a hydraulicbrake drive configured to accomplish parking braking of a vehicle wheelthat is not equipped with the electric parking brake, before thethickness loss is determined. The electronic control unit includes aseventh module 1007 configured to control the hydraulic brake drive.

The electronic control unit 100 further includes a sixth module 1006configured to provide instructions for determining the current thicknessof the monitored braking frictional pad on the basis of its thicknessloss and/or displaying the current thickness of the monitored brakingfrictional pad on a dashboard of the motor vehicle or transmitting it toa mobile device of the driver. For example, the instructions can beprovided to a displaying meter of the dashboard of the motor vehicle viaan existing bus of the motor vehicle or can be sent to the driver'smobile device such as a mobile phone or a notebook computer via wirelesscommunication, such as WIFI, a mobile network or the like.

The current thickness of the monitored braking frictional pad is thethickness of a new braking frictional pad of the same specification asthe monitored braking frictional pad minus the thickness loss of themonitored braking frictional pad.

A direction along which the piston driving part moves to the referenceposition is opposite to a direction along which the piston driving partmoves from the reference position to the stop position.

For the motor vehicle that is equipped with the electric mechanicalbrakes for the front and rear vehicle wheels, the electronic controlunit 100 further includes an eighth module 1008 which is configured toprovide instructions for: 1) first accomplishing parking braking of thefront vehicle wheels by their electric mechanical brakes, and thendetermining the thickness loss of the braking frictional pads of therear vehicle wheels via their electric mechanical brakes; or 2) firstaccomplishing parking braking of the rear vehicle wheels by theirelectric mechanical brakes, and then determining the thickness loss ofthe braking frictional pads of the front vehicle wheels via theirelectric mechanical brakes.

Furthermore, systems and methods or processes for monitoring wear of abraking frictional pad of a motor vehicle according to anotherembodiment of the present disclosure is explained below, wherein avehicle wheel is not provided with an electric parking brake or anelectric mechanical brake.

FIG. 7A is a schematic diagram illustrating that parking braking iscarried out by a brake piston driven by a hydraulic brake drive, whereina reference numeral 500 represents a brake disc, for example, the brakedisc 500L, 500FR, 500RL, or 500RR of FIG. 1; reference numerals 610 and620 represent braking frictional pads, such as the braking frictionalpads 610FL, 620FL; 610FR, 620FR; 61ORL, 620RL; 61ORR, 620RR of FIG. 1; areference numeral 320 represents a cylinder body of a brake wheelcylinder; a reference numeral 319 represents the brake piston of thebrake wheel cylinder. It should be understood by a person skilled in theart that the cylinder body 320 and the brake piston 319 can be acylinder body and a brake piston of a brake wheel cylinder for a vehiclewheel that is not equipped with an electric parking brake or an electricmechanical brake, or can be the cylinder body and the brake piston inthe embodiment as shown in FIGS. 2A and 2B. The cylinder body 320 isinstalled immovably relative to the brake disc 500, and when braking isrequired, the brake piston 319 can be driven by a pressurized brakefluid to move towards the braking frictional pad 610 such that underaction of a brake caliper (not shown by FIG. 7A), the two brakingfrictional pads 610, 620 can squeeze the brake disc 500 therebetween toapply a braking frictional force onto the brake disc 500.

It should be noted that a ring-shaped leak-proof feature is generallyprovided between the brake piston 319 and the cylinder body 320. Inaddition to preventing the brake fluid from leaking outwards through agap between the brake piston 319 and the cylinder body 320, thering-shaped leak-proof feature has another important function: whenbraking is not needed and the brake fluid is depressurized anddischarged from the brake piston 319, the ring-shaped leak-proof featurewould always drive the brake piston 319 to retreat slightly away fromthe braking frictional pad 610, which releases the brake piston frombeing in contact with the braking frictional pad 610. Generally, theextent of retreat is very tiny, cannot be seen by naked eyes, and isonly dependent on the elasticity of the ring-shaped leak-proof feature.Therefore, the extent of retreat (or can be referred to as the retreatdistance) can be regarded as a constant.

A volume space surrounded by the brake piston 319 and the cylinder body320 is in fluid communication with the brake fluid reservoir 400 via ahydraulic pipeline. Given that expansibility of the hydraulic pipelineis neglectable and the volume of the brake fluid in the motor vehicle isnot compressible, a change in the size of the volume space surrounded bythe brake piston 319 and the cylinder body 320 can result in a fluidlevel change of the brake fluid in the brake fluid reservoir 400. Asshown in FIG. 1, the brake fluid reservoir 400 is equipped with a fluidlevel sensor 410.

Now, assuming that the braking frictional pads 610 and 620 of FIG. 7Aare brand new ones; and that the braking frictional pads 610 and 620 ofFIG. 7B are used frictional pads after certain mileages of driving. Ifonly hydraulic braking is used, the brake fluid will be pressurized tofill the volume space surrounded by the brake piston 319 and thecylinder body 320 to drive the brake piston 319 to press the brakingfrictional pad 610 whenever braking is required. Therefore, after thebrake fluid is depressurized and discharged, the brake piston 319 willalways retreat by a tiny extent (under the action of the ring-shapedleak-proof feature). In the long term, because of the thinning of thebraking frictional pad 610, the volume space surround by the brakepiston 319 and the cylinder body 320 can become larger, beingrepresented by ΔV in FIG. 7A or 7B. ΔV is associated with a thicknessloss ΔL of the braking frictional pad 610 and thus will cause the fluidlevel in the brake fluid reservoir 400 to drop. Such dropping can begenerally detected or measured by the fluid level sensor 410. The fluidlevel sensor 410 is electrically connected to the electronic controlunit 100 to transmit detected or measured data to the electronic controlunit.

Under the assumption that no additional brake fluid is added into thebrake fluid reservoir or the amount of the brake fluid originally addedin the brake fluid reservoir is kept unchanged, the fluid level sensor410 can detect or measure the fluid level of the brake fluid reservoir410 of a motor vehicle in which a new braking frictional pad isinstalled, the detected or measured fluid level being recorded as areference fluid level; after certain mileage (such as 30000, 50000,80000 kilometers) of driving, the fluid level sensor 410 can againdetect or measure the current fluid level and determine the differencebetween the currently detected or measured fluid level and the recordedreference fluid level; at the same time, an already known thickness lossof a braking frictional pad can be used to determine an unknownthickness loss of other braking frictional pads. For instance, thereference fluid level can be an average of values of the fluid leveldetected or measured within a given period (for example, one week) afterthe new braking frictional pads are installed, to reduce the errorintroduced by a single detection or measurement.

Still referring to the embodiment as shown in FIG. 1, assuming that thethickness loss of the braking frictional pads of the rear vehicle wheelshas been determined by the method shown in FIG. 5, the thickness loss ofthe front vehicle wheels can be determined by the method illustrated inFIGS. 7A and 7B.

For instance, if a reference fluid level has been recorded, aftercertain mileage of driving, a volume difference ΔV_(L) between a volumeoccupied by a brake fluid corresponding to the reference fluid level anda volume occupied by the brake liquid corresponding to a current fluidlevel detected or measured by the fluid level sensor 410 can becalculated using the following equation:

ΔV _(L) =f2(D _(F) ,ΔL _(F) ,D _(R) ,ΔL _(R))

wherein D_(F) is a diameter of a brake piston of a brake wheel cylinderapplied to a front vehicle wheel and can be measured in advance; ΔL_(F)is a thickness loss of a braking frictional pad (i.e., a single brakingfrictional pad) of the front vehicle wheel; D_(R) is a diameter of abrake piston of a brake wheel cylinder applied to a rear vehicle wheeland can be measured in advance; ΔL_(R) is a thickness loss of a brakingfrictional pad (i.e., a single braking frictional pad) of the rearvehicle wheel; and f2 is a function depending on D_(F), ΔL_(F), D_(R),and AL_(R).

As an example (still referring to the embodiment as shown in FIGS. 2Aand 2B), AL_(R) has been determined with respect to the thickness lossof the braking frictional pad (the single braking frictional pad) of therear vehicle wheel using the method shown in FIG. 5. When braking iscarried out by a cylinder body of a wheel cylinder installed on a brakecaliper of a single respective rear vehicle wheel, a volume of the brakefluid additionally supplied into (due to the thickness loss ΔL_(R)) thecylinder body is:

2·1/4·πD _(R) ² ·ΔL _(R).

Therefore, when braking is carried out by cylinder bodies of wheelcylinders installed on brake calipers of two rear vehicle wheels, avolume of the brake fluid additionally supplied into (due to thethickness loss ΔL_(R)) the cylinder bodies is:

2·1/4·πD _(R) ²·2ΔL _(R).

Here, ΔL_(R) can be predetermined by the method shown in FIG. 5.

Similarly, when braking is carried out using wheel cylinders of twofront vehicle wheels , a volume of the brake fluid additionally suppliedinto (due to the thickness loss ΔL_(F)) the cylinder bodies is:

2·1/4·πD _(F) ²·2ΔL _(F).

Here, ΔL_(F) is unknown and the method shown in FIG. 5 is not applied.

Considering the volume of the brake fluid is not compressible, the fluidlevel will drop in the brake fluid reservoir 400 (that is to say, thebrake fluid contained in the brake fluid reservoir 400 will decrease)because additional brake fluid is sucked into the cylinder bodies of thebrake wheel cylinders of the front and rear vehicle wheels due to thethinning of the respective braking frictional pads. Moreover, in view offactors including brake disc wear, fluid pipeline loss and the like, acorrection factor γ (which can be determined in advance using multiplecalibration tests conducted during product development) will be added.Therefore, a volume difference ΔV_(L) of the brake fluid contained inthe brake fluid reservoir 400, which is caused by additional brake fluidbeing sucked into the cylinder bodies of the wheel cylinders of thefront and rear vehicle wheels due to the thinning of the respectivebraking frictional pads, can be calculated by the following equation:

ΔV _(L)=2·1/4·πD _(F) ²·2ΔL _(F)+2·1/4·πD _(R) ²·2ΔL _(R) +γ=πD _(F) ²·ΔL _(F) +πD _(R) ²ΔL_(R)+γ.

Using this equation, when ΔV_(L) can be detected or measured by thefluid level sensor 410 of the brake fluid reservoir 400 and D_(F),D_(R), and ΔL_(R) can be determined as discussed above, the thicknessloss ΔL_(F) of a single braking frictional pad of the front vehiclewheel can be determined by the following equation (2):

ΔL _(F)=(ΔV _(L) −πD _(R) ² ·ΔL _(R)−γ)/(πD_(F) ²)   (2).

The equation (2) can have a variant of ΔL_(x)=(ΔV_(L)−πD₀²·ΔL₀−γ)/(πD_(x) ²), wherein ΔV_(L) is the volume difference of thebrake fluid in the brake fluid reservoir 400 detected by the fluid levelsensor 410, ΔL₀ is a known thickness loss of a single braking frictionalpad, D₀ is a diameter of a brake piston located at a brake caliperassociated with the single braking frictional pad (and can be measuredin advance); γ is a correction factor (which can be determined inadvance); D_(x) is a diameter of a brake piston located at a brakecaliper associated with a single braking frictional pad whose thicknessloss is to be determined (and this diameter can be measured in advance);and ΔL_(x) is the thickness loss of the single braking frictional pad tobe determined.

As already explained, when only hydraulic braking is carried out, thebrake piston will retreat by a little constant distance after it drivesthe braking frictional pad to accomplish the hydraulic braking. Itshould be understood by a person skilled in the art that the abovemethod disregards the effect of the little retreating constant distanceon the fluid level change. This is because the fluid level change causedby the retreat is relatively uniform, and has the same effect onto a newbraking frictional pad and a used braking frictional pad. Usually, thiseffect is too tiny to change the fluid level in the brake fluidreservoir 400.

The method for monitoring the wear of the braking frictional padaccording to the present disclosure is advantageous in that a thicknessloss of a braking frictional pad of a vehicle wheel that is not equippedwith an electric parking brake can be calculated or deduced by acomparison between a fluid level detected or measured by the fluid levelsensor 410 after certain mileage of driving and a reference fluid level.It would be understood by a person skilled in the art that for a motorvehicle equipped with both electric parking brakes (for example, at itsfront vehicle wheels or rear vehicle wheels only) and electricmechanical brakes, the above-mentioned method for calculating thethickness loss of a braking frictional pad of a vehicle wheel that isnot equipped with the electric mechanical brake can also be applicable.

For example, in the embodiment shown in FIG. 1, although ΔL_(R) of therear vehicle wheel's braking frictional pad can be determined by theelectric parking brake configured for the rear vehicle wheel, ΔL_(R) ofthe rear vehicle wheel's braking frictional pad can be alternatively oradditionally determined by a resistance sensor configured for the rearvehicle wheel. For instance, the resistance sensor can be at leastpartially embedded in the rear vehicle wheel's braking frictional pad,such as a braking frictional pad located at one side of a brake disc. Inthis way, depending on the wear of the braking frictional pad along itsthickness direction, different resistance values can be output by theresistance sensor. Furthermore, the resistance sensor can beelectrically connected to the electronic control unit 100 to transmitresistance value data. The resistance values can also be output by theresistance sensor intermittently. That is to say, only when the wear ofthe braking frictional pad exceeds a preset value, data reflecting theresistance change can be output (for example, several wear states can beset to correspond to several outputs of the resistance change). For abraking frictional pad quipped with the resistance sensor, advancedcalculation can enable each resistance value output to be associatedwith a given (or the current) thickness of the braking frictional pad ora thickness loss of the braking frictional pad. Further, taking a rearvehicle wheel having braking frictional pad equipped with such aresistance sensor for example, when the electronic control unit 100receives an output of the resistance change from the resistance sensor,a thickness loss of the braking frictional pad can be correspondinglydeduced. In this way, a thickness loss of a braking frictional padequipped for a vehicle wheel (for example, the front vehicle wheel shownin FIG. 1) that is not equipped with an electric parking brake can bededuced from the equation (2) again.

FIG. 8 is a flow chart illustrating a method or process according toanother embodiment of the present disclosure for monitoring wear of abraking frictional pad. A person skilled in the art would understandthat any method and process or their steps explained herein can be codedas programs to be stored in the memory of the electronic control unit100 and called and executed as required.

First, in step S100, wear monitoring is initiated. For example, for amotor vehicle in which only front vehicle wheels or rear vehicle wheelsare equipped with electric parking brakes or electric mechanical brakes,or for a motor vehicle in which only front vehicle wheels or rearvehicle wheels are equipped with resistance sensors, the methoddescribed with respect to FIG. 5 can be used to determine a thicknessloss of a braking frictional pad of the front or rear vehicle wheels oran output reflecting the sensor's resistance change can be transmittedby the resistance sensor to the electronic control unit 100 to determinea thickness loss of a respective braking frictional pad. Then, in stepS110, whether the motor vehicle has been stopped stably or not isdetermined. A collection of information from various existing sensors ofthe motor vehicle can be used to determine whether or not the motorvehicle has been stopped stably on a horizontal or substantiallyhorizontal plane. There, the “substantially horizontal plane” may meanthat such plane is off by a small angle range from the sea horizontalplane, for example ±5 degrees or ±10 degrees. The purpose for doing sois to ensure that the fluid in the brake fluid reservoir 400 ishorizontally leveled or substantially leveled thus each fluid levelmeasurement carried out by the fluid level sensor 410 can be accuratelyrecorded and compared. If the determination of step S110 is that themotor vehicle is stopping on the horizontal or substantially horizontalplane, the method or process goes to step S140; otherwise, the method orprocess ends. In step S140, a current fluid level of the brake fluid inthe brake fluid reservoir 400 can be measured by the liquid sensor 410,and can be compared with a reference fluid level to determine theabsolute value of the difference therebetween and thus to determine avolume change of the brake fluid, wherein the reference fluid level hasbeen measured when a new braking frictional pad was installed or thereference fluid level has been measured when the thicknesses of all ofthe braking frictional pads are known. Then, in step S150, a thicknessloss of a braking frictional pad configured for a vehicle wheel that isnot equipped with the electric parking or mechanical brake is determinedby the equation (2) using parameters including the thickness loss of thefront or rear vehicle wheel's braking frictional pad which has beenalready determined in step S100, the volume difference of the brakefluid already determined in step S140, a diameter of the respectivebrake piston.

Then, in step S600, if the thickness loss of the braking frictional paddetermined in step S150 is greater or equal to a given value, the methodor process goes to step S700. For instance, in step S700, an indicatorin a passenger compartment can be activated to visually and/or audiblyalert a user sitting in the passenger compartment; and/or some functionsof the motor vehicle can be limited, for example, the motor vehicle isallowed to run only at a restricted speed such as a maximum speed of 50kilometers per hour, to alert the driver. If the thickness loss of thebraking frictional pad determined in step S150 is less than the givenvalue, the method or process goes to step S800 and thickness loss datacan be sent to a dashboard or the driver's mobile device to be displayedas status information.

The present disclosure further proposes another method for monitoringwear of a braking frictional pad of a motor vehicle, wherein the motorvehicle includes a brake device acting on each vehicle wheel. The brakedevice has a brake disc that rotates with the vehicle wheel, and abraking frictional pad that is non-rotatable relative to the vehiclewheel and linearly movable parallel to a rotational axis of the vehiclewheel. The motor vehicle further includes a hydraulic brake drive and abrake fluid reservoir in fluid communication with the hydraulic brakedrive. The hydraulic brake drive has a cylinder body installed to themotor vehicle's body and a brake piston movable linearly within thecylinder body. The brake piston of the hydraulic brake drive isconfigured to act on the braking frictional pad, where a brake fluidsupplied by the brake fluid reservoir is provided to the brakingfrictional pad, moving the braking frictional pad to contact the brakedisc.

The method includes:

-   stopping the motor vehicle stably in a horizontal or substantially    horizontal plane;-   determining a current fluid level in the brake fluid reservoir;-   determining a volume difference of the brake fluid in the brake    fluid reservoir by comparing a preset reference fluid level and the    determined current fluid level; and-   when a thickness loss of a braking frictional pad equipped for one    of a front vehicle wheel and a rear vehicle wheel is known,    determining a thickness loss of a braking frictional pad equipped    for the other of the front vehicle wheel and the rear vehicle wheel    by the volume difference of the brake fluid in the brake fluid    reservoir and a diameter of the brake piston of the respective brake    device.

Additionally, when the thickness loss of the braking frictional padequipped for one of the front vehicle wheel and the rear vehicle wheelis known, the thickness loss of the braking frictional pad equipped forthe other of the front vehicle wheel and the rear vehicle wheel isdetermined using an equation as follows:

ΔL _(X)=(ΔV _(L) −πD ₀ ² ·ΔL ₀−γ)/(πD _(X) ²)

wherein ΔL_(x) is the thickness loss of the monitored braking frictionalpad, D₀ is a diameter of the brake piston having the known thicknessloss, D_(x) is a diameter of a brake piston associated with themonitored braking frictional pad, ΔV_(L) is the volume difference of thebrake fluid in the brake fluid reservoir, γ is an empirical constant.

Furthermore, the present disclosure proposes another system formonitoring wear of a braking frictional pad of a motor vehicle. As shownin FIG. 9, the system includes:

-   a fluid level sensor 410 disposed in a brake fluid reservoir 400;    and-   an electronic control unit 100 connected to the fluid level sensor    410 to receive fluid level data, wherein the electronic control unit    100 includes:-   a first module 2001 configured to provide instructions for stopping    the motor vehicle stably in a horizontal or substantially horizontal    plane;-   a second module 2002 configured to provide instructions for    determining a current fluid level in the brake fluid reservoir;-   a third module 2003 configured to provide instructions for    determining a volume difference ΔV_(L) of the brake fluid in the    brake fluid reservoir by comprising a preset reference fluid level    and the determined current fluid level; and-   a fourth module 2004 configured to provide instructions for    determining a thickness loss of a braking frictional pad equipped    for one of a front vehicle wheel and a rear vehicle wheel by the    volume difference of the brake fluid in the brake fluid reservoir    and a diameter of the brake piston of the respective brake device    when a thickness loss of a braking frictional pad equipped ed for    the other of the front vehicle wheel and the rear vehicle wheel is    known.

The electronic control module 100 further includes:

-   a fifth module 2005 configured to provide instructions for operating    a drive motor to move a piston driving part to a preset reference    position after the motor vehicle has been parked;-   a sixth module 2006 configured to provide instructions for, after    the fifth module 2005, operating the drive motor again to move the    piston driving part from the reference position to a stop position    where it contacts the brake piston, wherein the stop position    corresponds to a parking braking position of the brake piston; and-   a seventh module 2007 configured to provide instructions for    determining a distance between the reference position and the stop    position and comparing the determined distance with a preset    reference distance to determine a thickness loss of the monitored    braking frictional pad as the known thickness loss.

The electronic control unit 100 further includes an eighth module 2008configured to provide instructions for alerting a driver of the motorvehicle to replace the braking frictional pad (to be monitored) with anew one when the thickness loss of any braking frictional pad exceeds agiven value.

Although some specific embodiments and/or examples of the presentdisclosure are described here, they are given for illustrative purposesonly and cannot be deemed to restrict the scope of the presentdisclosure in any way. Furthermore, it should be understood by a personskilled in the art that the embodiments and/or examples described herecan be combined. Without departing from the spirit and scope of thepresent disclosure, various replacements, modifications, andalternations can be carried out.

What is claimed is:
 1. A method for monitoring wear of a braking frictional pad of a motor vehicle, the motor vehicle comprising vehicle wheels, a brake device acting on each vehicle wheel, the brake device including a braking frictional pad configured to be non-rotatable relative to the vehicle wheel and be linearly movable parallel to a rotational axis of the vehicle wheel, the motor vehicle also comprising a hydraulic brake drive and a brake fluid reservoir in fluid communication with the hydraulic brake drive, the hydraulic brake drive including a linearly movable brake piston configured to act on the braking frictional pad to brake the vehicle wheel, the method comprising: stopping the motor vehicle stably on a horizontal or substantially horizontal plane; determining a current fluid level in the brake fluid reservoir; determining a volume difference (ΔV_(L)) of the brake fluid in the brake fluid reservoir by comparing the determined current fluid level with a reference fluid level; and when a thickness loss of a braking frictional pad equipped for a front vehicle wheel or a rear vehicle wheel is known, determining a thickness loss of a braking frictional pad equipped for the other front vehicle wheel or the other rear vehicle wheel based on the volume difference of the brake fluid in the brake fluid reservoir and a diameter of the brake piston of the respective brake device.
 2. The method according to claim 1, wherein the thickness loss of the braking frictional pad equipped for the other front vehicle wheel or the other rear vehicle wheel is determined using the following equation: ΔL _(X)=(ΔV _(L) −πD ₀ ² ·ΔL ₀−γ)/(πD _(X) ²) wherein ΔL_(x) is the thickness loss of the braking frictional pad to be determined, D₀ is a diameter of the brake piston associated with the braking frictional pad having the known thickness loss, D_(x) is a diameter of a brake piston associated with the braking frictional pad with the thickness loss to be determined, ΔV_(L) is the volume difference of the brake fluid in the brake fluid reservoir, γ is an empirical constant.
 3. The method according to claim 1, wherein the reference fluid level is a fluid level in the brake fluid reservoir measured when the motor vehicle stays stably in a horizontal or substantially horizontal plane and when the motor vehicle is installed with brand new braking frictional pads or braking frictional pads having known thickness for all vehicle wheels.
 4. The method according to claim 3, wherein after the reference fluid level has been measured and before any subsequent determination of fluid level in the brake fluid reservoir, no additional brake fluid is added into the brake fluid reservoir.
 5. The method according to claim 4, wherein the known thickness loss of the braking frictional pad is determined by a resistance sensor disposed within the braking frictional pad.
 6. The method according to claim 4, wherein an electric parking brake or an electric mechanical brake is equipped for the braking frictional pad having the known thickness loss.
 7. The method according to claim 6, wherein the electric parking brake or the electric mechanical brake comprises a drive motor, and a piston driving part driven by the drive motor to be linearly movable; when the motor vehicle is braked, the brake piston is driven by the piston driving part to contact and move the braking frictional pad.
 8. The method according to claim 7, further comprising: operating, after the motor vehicle has been parked, the drive motor to move the piston driving part to a preset reference position; continuing to operate the drive motor to move the piston driving part from the reference position to a stop position where the piston driving part drives the brake piston to contact the braking frictional pad, wherein the stop position corresponds to a parking braking position of the brake piston; determining a distance between the reference position and the stop position; and calculating a difference between the determined distance and a predetermined reference distance to determine a thickness loss of the associated braking frictional pad as the known thickness loss.
 9. The method according to claim 8, wherein the reference distance is a travel distance of the piston driving part from the reference position to the stop position, when the thickness of the associated braking frictional pad is known or the associated braking frictional pad is brand new, and when the motor vehicle is in parking braking.
 10. The method according to claim 9, wherein the known thickness loss is absolute value of the difference between the determined distance and the reference distance.
 11. The method according to claim 10, further comprising alerting a driver of the motor vehicle to replace the braking frictional pad with a new one when the thickness loss of the braking frictional pads exceeds a threshold.
 12. The method according to claim 11, wherein the electric parking brake or the electric mechanical brake further comprises a reducer mechanism connected to an output shaft of the drive motor, and a linear movement mechanism driven by the reducer mechanism, the reducer mechanism is configured to provide a lock function in braking to prevent the piston driving part from moving freely, and the piston driving part is configured to be driven by the linear movement mechanism to linearly move.
 13. The method according to claim 12, wherein the linear movement mechanism is a thread-screw mechanism that includes a core shaft connected to the output shaft of the drive motor, the core shaft comprising external threads on its outer surface, and the piston driving part is a threaded part having internal threads that engage the external threads of the core shaft and is configured to be non-rotatable relative to the core shaft, and be axially movable back and forth along the core shaft.
 14. The method according to claim 13, wherein the distance between the reference position and the stop position is at least dependent on a rotation speed of the output shaft of the drive motor, a transmission ratio of the reducer mechanism, a pitch of thread of the thread-screw mechanism, and a time duration during which the drive motor is turned on.
 15. The method according to claim 9, wherein before the known thickness loss is determined, parking braking of a vehicle wheel that is not equipped with the electric parking brake is carried out by the hydraulic brake drive.
 16. The method according to claim 15, wherein a direction along which the piston driving part moves to the reference position is opposite to a direction along which the piston driving part moves from the reference position to the stop position.
 17. A system for monitoring wear of a braking frictional pad of a motor vehicle, the motor vehicle comprising vehicle wheels and a brake device acting on each of the vehicle wheels, the brake device including a braking frictional pad configured to be non-rotatable relative to the vehicle wheel and be linearly movable parallel to a rotational axis of the vehicle wheel, the motor vehicle also comprising a hydraulic brake drive and a brake fluid reservoir in fluid communication with the hydraulic brake drive, the hydraulic brake drive including a linearly movable brake piston which is configured to act on the braking frictional pad to brake the vehicle wheel, the system comprising: a fluid level sensor provided in the brake fluid reservoir; and an electronic control unit connected to the fluid level sensor to receive fluid level data, comprising: a first module configured to provide instructions for stopping the motor vehicle stably on a horizontal or substantially horizontal plane; a second module configured to provide instructions for determining a current fluid level in the brake fluid reservoir; a third module configured to provide instructions for determining a volume difference (ΔV_(L)) of the brake fluid in the brake fluid reservoir based on a preset reference fluid level and the determined current fluid level; and a fourth module configured to provide instructions for, when a thickness loss of a braking frictional pad associated with a front vehicle wheel or a rear vehicle wheel is known, determining a thickness loss of a braking frictional pad associated with the other front vehicle wheel or the other rear vehicle wheel based on the volume difference of the brake fluid in the brake fluid reservoir and a diameter of the brake piston of the respective brake device.
 18. The system according to claim 17, wherein the fourth module is configured to determine the thickness loss of the braking frictional pad associated with the other front vehicle wheel or the other rear vehicle wheel using an equation as follows: ΔL _(X)=(ΔV _(L) −πD ₀ ² ·ΔL ₀−γ)/(πD _(X) ²) wherein ΔL_(x) is the thickness loss of the braking frictional pad to be determined, D₀ is a diameter of the brake piston associated with the braking frictional pad having the known thickness loss, D_(x) is a diameter of a brake piston associated with the braking frictional pad with the thickness loss to be determined, ΔV_(L) is the volume difference of the brake fluid in the brake fluid reservoir, γ is an empirical constant.
 19. The system according to claim 17, wherein the reference fluid level is a fluid level in the brake fluid reservoir, when the motor vehicle stops on a horizontal or substantially horizontal plane and the thickness of each braking frictional pad is known or each braking frictional pad is brand new.
 20. The system according to claim 19, wherein no additional brake fluid is added into the brake fluid reservoir of the motor vehicle. 